Polycrystalline alumina (PCA) arc tubes have been employed for many years in high pressure sodium (HPS) lamps. Recently, such arc tubes have found important application for metal halide lamps where non-cylindrically shaped arc tubes have shown advantages for improved efficacy. Such non-cylindrical shapes include elliptical and bulgy geometries.
Prior art methods for forming these arc tubes have employed cold isostatic pressing; slip casting; tape casting, injection molding; blow molding; gel casting or extrusion. While these methods have worked well with simple cylindrical shapes, intricately shaped, one-piece and complex parts are often limited by the difficult and sometimes impossible removal of the structural core material used to define the internal geometry. Further, contamination caused by contact with core materials often leads to problems, especially in optical ceramics where high purity is a requirement. Extrusion and blow molding of hollow ceramic bodies can show warpage due to the plastic flow of the wetted extrusion mixture, distorting or even collapsing the cavity or allowing variable wall thickness due to diameter expansion or variation in the material stiffness. Pressed or cast one-piece parts are limited in shape due to the inability to remove the mandrel or core. If cast without a core, the interior geometry is variable, often requiring expensive diamond grinding of the sintered parts.
It would be an advance in the art to provide a production-viable method of manufacturing complex-shaped arc tubes of PCA suitable for use as the discharge vessels of HPS and metal halide lamps.